Although most optical communication systems today use binary On-Off Keying (OOK), differential phase shift keying (DPSK) has been demonstrated experimentally as a very attractive alternative to OOK for long haul optical communications. In long haul optical communication systems, for a given bit-error ratio (BER), binary DPSK requires nearly 3 dB lower optical signal-to-noise ratio (OSNR) than OOK, enabling extended reach. Differential quadrature-phase-shift keying (DQPSK) offers all the advantages of multilevel encoding which increases spectral efficiency, improved tolerance to chromatic dispersion (CD) and polarization-mode dispersion (PMD), and relaxed components bandwidth requirements without incurring a power penalty with respect to OOK.
There are basically three modulation formats in a telecommunication system, which are On-Off keying (OOK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). On-Off keying (OOK) is using the signal to modulate the amplitude of a carrier wave laser, while the Frequency Shift Keying (FSK) or the Phase Shift Keying (PSK) is using the signal to modulate either the frequency or the phase of a carrier wave laser instead of modulating the amplitude that is the OOK scheme. Both FSK and PSK systems depend on highly frequency-stabled carrier wave laser and local oscillator laser with frequency, phase and polarization matched with the carrier wave laser to get the information.
DPSK system is slightly modified PSK system. The term ‘differential’ means that the information is encoded into the bit-to-bit phase change rather than into an absolute phase value. This avoids both the frequency or phase ambiguity inherent to a FSK or PSK system and the necessity of very stable local oscillator laser and carrier wave laser. Due to the lack of an absolute phase reference in direct-detection receiver, the phase of the preceding bit is used as a relative phase reference for demodulation.
However, DPSK and DQPSK modulation formats require a delay interferometer for demodulation at the receiver. This device converts the phase-coded information into detectable intensity information relying on the phase difference between adjacent bits. Realizations of the delay interferometer typically include integrated planar waveguide and fiber-based interferometers. The path-length difference of the delay interferometer can be fine-tuned for phase control using either an integrated thermal heater or a piezoelectric transducer. Birefringence in the fibers must also be very low, because in the presence of birefringence, perfect centering of the interferometer cannot be achieved simultaneously for both polarizations, resulting in system penalties.
A delay interferometer, such as Mach-Zehnder interferometer, comprises one splitter in series with a combiner, the two optical paths between the splitter and the combiner being of different lengths that provide a well-defined delay for two optical signals corresponding to the duration of a bit signal. At the output side of the delay interferometer the two signals are detected by a balanced receiver in order to extract information from the received DPSK encoded optical signals. It is very important that the delay for two optical signals be independent of the state of polarization of the two optical signals.
In U.S. Pat. No. 6,271,959 there is disclosed an optical frequency demodulation technique for extracting overhead signal from an optical channel carrying both payload and overhead signals. The technique makes use of asymmetrical Mach-Zehnder interferometry (AMZI) in an optical frequency demodulator to extract the overhead signal information. The optical frequency demodulator is composed of a tuned asymmetric Mach-Zehnder interferometer for extracting the overhead signal by frequency discrimination, a balanced receiver pair for converting the overhead signal into an electrical signal and a low-pass filter for eliminating high frequency components from recovered overhead signal information. There is no indication, however, in this patent that a similar technique could be used to demodulate DPSK signal.
In European Patent Application No. EP 1 335 510 A1 there is disclosed a delay-line interferometer based on single-mode fibers. The device is composed of two beam splitters and well-defined equal total bending angle optical paths between the two splitters to ensure equal polarization in both optical paths.
The bending of optical fiber will introduce birefringence in the fiber because of the elasto-optic effect. It is difficult to keep the total bending angle equal for both optical paths between the two splitters, especially if one tries to reduce the size of the device. The other problems associated with the delay-line interferometer of EP 1 335 510 A1 are reliability of the bended optical fibers and the ability of tuning the optical channel center with the laser frequency when environment temperature changes occur.
In yet another prior art U.S. Pat. No. 6,563,971 there is disclosed an optical fiber Mach-Zehnder interferometer employing miniature bends. The device is made of two couplers with tapered optical paths miniature bends in between in order to reduce the size of an optical fiber Mach-Zehnder interferometer. One major problem associated with the tapered miniature bends is the large birefringence that prevents this technique to be used in the delay interferometer for DPSK demodulation application. The other problem with this technique is the inability to coat the fiber with a metal film in the tapered bend region because of induced high losses.
Therefore, there is a need for an optical delay interferometer with low-loss, low birefringence, fast phase-tuning capability, able to work in wide spectral range, as well as being highly reliable and preferably of small size.